Lithium secondary batteries have been widely used as power sources of portable devices after they have emerged as small, lightweight, and high-capacity batteries since 1991. Recently, in line with the rapid development of electronics, communications, and computer industries, camcorders, mobile phones, and notebook PCs have appeared and undergone continuous and remarkable development. Accordingly, the demand for lithium secondary batteries as a power source for driving these portable electronic information and communication devices has increased day by day.
Lithium secondary batteries have limitations in that their lifetime rapidly decreases as charge and discharge are repeated. In particular, the above limitations are more severe at high temperature. The reason for this is due to a phenomenon that occurs when an electrolyte is decomposed or an active material is degraded due to moisture in the battery or other effects, and the internal resistance of the battery increases.
In order to address the above limitations, a technique of coating the surface of a cathode active material with an oxide of metal, such as magnesium (Mg), aluminum (Al), cobalt (Co), potassium (K), sodium (Na), and calcium (Ca), by a heat treatment has developed. Also, research to improve energy density and high-rate characteristics by adding TiO2 to a LiCoO2 active material has been conducted.
However, limitations, such as lifetime degradation or gas generation due to the decomposition of the electrolyte during charge and discharge, have not been fully resolved yet.
Also, a technique of coating the surface of a cathode active material with a coating agent, such as metal oxide or a metal fluoride compound, has recently been developed. This coating technique is a method in which the coating agent is adhered to the surface of the cathode active material by using a sol-gel method or colloidal method using an aqueous-based or organic-based material as a solvent and a heat treatment is then performed.
With respect to a wet process in which surface coating is performed using the solvent, electrochemical properties of structurally stable LiCoO2 may be improved, but, with respect to Li[Ni1-xMx]O2 or Li[NixCo1-2xMnx]O2, a surface modification effect may not be obtained due to structural changes or electrochemical performance may be significantly reduced.
Furthermore, in the case that impurities are present on the surface of a cathode active material during a process of fabricating an electrode of a lithium secondary battery, aging in a step of preparing an electrode slurry during the process of fabricating an electrode of a lithium secondary battery may not only be affected, but may also cause a swelling phenomenon in the lithium secondary battery by reacting with an electrolyte solution that is injected into the lithium secondary battery.
Therefore, there is an urgent need to develop a cathode active material for a lithium secondary battery which is structurally stable and may minimize the amount of lithium impurities included in the cathode active material.